(a) Field of the Invention
The present invention relates to a method for collecting metallic contaminants from a wafer and, more particularly to a technique for collecting metallic contaminants from a semiconductor wafer by using a new and improved sampling vessel.
(b) Description of the Related Art
An atomic absorption method (AAS) or an inductive coupling plasma mass spectrometry (ICP-MS) is used as a method for analyzing the amount of metallic contaminants on the surface of a semiconductor wafer. The sample solution used for these analyses is prepared by the steps shown in FIGS. 1A to 1C.
An oxide film 21 formed on the surface of a semiconductor wafer 20 is dissolved by spray of hydrofluoric acid (HF) steam 23 onto the surface of the oxide film 21, an shown in FIG. 1A, to thereby remove the oxide film 21 and expose a silicon surface of the wafer 20, as shown in FIG. 1B. Subsequently, a droplet 14 of a collecting liquid for collecting metallic contaminant is dropped onto the exposed silicon surface of the wafer 20, as shown in FIG. 1C, followed by scanning of the silicon surface by the collecting droplet 14 to collect metallic contaminants 22 into the droplet 14. For instance, the metallic contaminants 22 on the wafer 20 are collected by a spiral scanning (FIG. 2A), or reciprocal scanning (FIG. 2B or FIG. 2C) of the droplet 14. The collecting droplet 14 is recovered by a pipette etc. together with the metallic contaminants 22 as a sample solution, transferred to a dish or vessel for the analysis, and analyzed for the amount of metallic contaminants by the atomic absorption method (AAS) or inductive coupling plasma mass spectrometry (ICP-MS).
During the step of dissolving the oxide film 21 on the silicon surface by HF steam 23, HF droplets may be formed on the silicon surface if the oxide film 21 to be removed is thick to thereby require a large length of time for spray of the HF steam 23 The HF droplets thus formed may move from the main surface of the wafer 20 toward the back surface thereof if the number of HF droplets thus formed is large. In this case, the HF droplets cannot be entirely recovered into the collecting droplet 14, which results in an error in the measurement of the metallic contaminants. A similar situation may occur when the scanning by the collecting droplet 14 is effected at the side surface or back surface of the wafer.
Moreover, the HF droplets formed on the surface of the wafer 20 from the HF steam 23 dilute the collecting droplet 14 to lower the concentration of the metallic contaminants in the collecting droplet 14, thereby lowering the accuracy of the measurement therefrom,